122 PALEONTOLOGY OF ILLINOIS. 



position would require very important modifications of the ordinary apparatus 

 by which their movements were effected. 



In their surface markings the spines under consideration resemble those of 

 Oracanthus, and it is probable one or more species have been described under 

 that name that more properly belong with those on which the genus Drepana- 

 canthus is now proposed; but the typical species of Oracanthus, such as 0. mil- 

 leri, Agassiz, 0. vetustus, Leidy, 0. pnigeus, Nob., were more broadly conical in 

 form and were composed of a cartilaginous center, covered with a thin crust of 

 bone and enamel, which was solid only at the tip; and we have no evidence that 

 any of the spines of this character were curved forward at the apex or were 

 toothed along the posterior border. Tn Drepanacanthus, on the contrary, the 

 spine was composed of dense, fibrous, bony tissue, with a pulp cavity of mod- 

 erate size, and the surface was not all covered with an enamel coating, but 

 thickly studded with spheroidal tubercles of black, glass-like enamel, and was 

 strongly curved forward at the summit. 



The figure and description of Oracanthus pustuhsus, (Agass. Poiss. Foss.,Vol. 

 3, p. 15, Atlas,Vol. 3, Tab. 2, figs. 3 and 4,) although representing only a frag- 

 ment, indicate a spine very different in its aspect and structure from 0. milleri, 

 and the other species enumerated above, and one much more like those which 

 we have designated by the name of Drepanacanthus. So little of 0. pustulosus 

 is preserved that it is not easy to decide whether, like these, it was heterostrophe 

 or not; but the greater relative thickness of bone on what was regarded as the 

 posterior side of the cavity, and the crowding of the tubercles over the exterior 

 surface of the same region, might lead us to suspect that the convex side was 

 the posterior, and that it was heterostrope. The tubercles of 0. pustulosus are 

 not fully described by M. Agassiz, but, judging from the figure, they are 

 spheroidal in form and ornamented by sharp carinations, just as are those of a 

 species Drepanacanthus now before us. If so, they are strikingly different from 

 those of the typical species of Oracanthus, which are conical, with a more or 

 less acute summit and a broad base, from which often radiate many radicles of 

 enamel. These reach up on to the sides of the cones, and give them the stel- 

 late appearance which they usually, though not constantly, present. 



If other and more complete specimens of 0. pustulosus should confirm the 

 conjectures we now make in reference to its curvature and ornamentation, it 

 would be a well-marked, and even typical species of the genus Drepanacanthus. 



Drepanacanthus anceps, N. and W. 



PI. XII, Fig. 8. 



Spine small, much curved and compressed throughout its 

 entire length; section lenticular, with acute edges; width near 



